Micro Mechanisms and Mechanics of Ductile Fracture Separation

Abstract

Ductile fracture occurs a result of the nucleation, growth and coalescence of voids. The rate at which these processes take place in mild steel has been studied under various states of stress and strain. The void spacings, as described by the parameter Average Nearest Neighbour Distance (A.N.N.D.), and the void sizes have been measured at applied plastic strains ranging from zero to 0.3 true strain, The voids were Produced in uniaxially stressed perallel tensile bars, uniaxially stressed single and multiple notched tensile bars, and in three point bend specimens. The void spacing was found to be a linear function of strain. The variation of spacing with strain under conditions of uniaxial tensile testing is in agreement with the observations made near notch tips once corrections have been made for the differences in strain concentrations. The void growth increases exponentially with the triaxial stress component of the stress state, which is in agreement with the available theoretical predictions, although these predictions were found to give an over- estimation of void growth rates. It has been demonstrated that the occurence of a ductile fracture is controlled more significantly by the amount of plastic strain than the magnitude of the triaxial component of the stress. The implications of this conclusion have been discussed in terms of the mechanisms of ductile fracture and the methods used to design against failure.